In radar (or sonar or lidar) processing to identify targets, the radar signals which are transmitted are ideally reflected by the desired target, and returned to the radar system for identification and ranging. One of the problems to which most radar systems are subject is that of clutter. Clutter is caused by undesired targets, such as trees in a land-based system or waves in a maritime system, which undesired targets are relatively near the radar system, or otherwise located so as to generate an undesired radar return signal. Radar system return signals are often said to follow an “inverse fourth-power” law, in that the strength of the return signal becomes smaller in proportion to the fourth power of the range of the target. A concomitant is that nearby clutter will produce target returns which are very much greater than the returns from small or inconspicuous targets at substantial range. Clutter can occur at any range, however, and tends to obscure desired targets at that range.
Schemes have been developed for reducing the effects of clutter. Moving-target indication (MTI) is a system which tends to suppress or cancel those return signals which exhibit the same return signal phase from pulse to pulse, which generally applies to nonmoving targets. Doppler filtering is a scheme in which the returned radar signal is applied to an array of frequency bins, which separate the signals in accordance with how fast the target is moving. The clutter signals tend to be grouped in the lowest-frequency bins, and can be processed independently from those returns showing higher frequencies indicative of greater speed.
Unfortunately, slow-moving targets, such as helicopters, which are near the radar system, may have return signals which are near the locations in which clutter occurs, and may have target velocities which are low or in the same range as that of clutter. Doppler filtering is not fully effective in extracting target information under such conditions.
Improved resolution of slow-moving targets is desired in the presence of clutter.